The present invention is generally directed to an implantable device for monitoring the progression or regression of heart disease. The present invention is more particularly directed to such a device which identifies heart sounds and electrogram features from heart sound and electrogram signals representing emitted sound and electrical activity of a patient""s heart and determines time intervals between selected heart sounds and electrogram features indicative of the progression or regression of the heart disease.
More people die of heart disease than any other single cause. One common form of heart disease is congestive heart failure.
Congestive heart failure (CHF) is a debilitating, end-stage disease in which abnormal function of the heart leads to inadequate blood flow to fulfill the needs of the body""s tissues. Typically, the heart loses propulsive power because the cardiac muscle loses capacity to stretch and contract. Often, the ventricles do not adequately fill with blood between heartbeats and the valves regulating blood flow may become leaky, allowing regurgitation or backflow of blood. The impairment of arterial circulation deprives vital organs of oxygen and nutrients. Fatigue, weakness, and inability to carry out daily tasks may result.
Not all CHF patients suffer debilitating symptoms immediately. Some may live actively for years. Yet, with few exceptions, the disease is relentlessly progressive.
As CHF progresses, it tends to become increasingly difficult to manage. Even the compensatory responses it triggers in the body may themselves eventually complicate the clinical prognosis. For example, when the heart attempts to compensate for reduced cardiac output, it adds muscle causing the ventricles to grow in volume in an attempt to pump more blood with each heartbeat. This places a still higher demand on the heart""s oxygen supply. If the oxygen supply falls short of the growing demand, as it often does, further injury to the heart may result. The additional muscle mass may also stiffen the heart walls to hamper rather than assist in providing cardiac output.
Current standard treatment for heart failure is typically centered around medical treatment using ACE inhibitors, diuretics, and digitalis. It has also been demonstrated that aerobic exercise may improve exercise tolerance, improve quality of life, and decrease symptoms. Only an option in 1 out of 200 cases, heart transplantation is also available. Other cardiac surgery is also indicated for only a small percentage of patients with particular etiologies. Although advances in pharmacological therapy have significantly improved the survival rate and quality of life of patients, patients who are refractory to drug therapy, have a poor prognosis and limited exercise tolerance. Cardiac pacing has been proposed as a new primary treatment for patients with drug-refractory CHF.
By tracking the progression or regression of heart disease, such as CHF, more closely, treatments could be managed more effectively. Commonly, patients with heart disease have an implanted cardiac stimulation device. Hence, it would be advantageous if the implanted cardiac stimulation device were able to aid in the tracking of the progression or regression of the heart disease. While some devices have been proposed to track a patient""s heart condition, these devices have relied upon sensing activity and/or respiration of the patient. Unfortunately, this requires additional complexity to an already complex device. The present invention addresses the issues of tracking heart disease progression or regression by making use of existing circuitry commonly available in implantable cardiac stimulation devices.
The present invention provides a system and method, for use in an implantable cardiac device, for monitoring a predetermined characteristic of heart sound to determine the progression or regression in heart disease, such as congestive heart failure.
A sensing circuit produces an electrogram signal indicative of the electrical activity of the patient""s heart. A sound, or acoustic, sensor produces a phonocardiogram representing the sounds omitted from the patient""s heart. A processor determines time intervals between selected heart sounds in the phonocardiogram and morphological features in electrogram. Relative changes in the time intervals or the amplitude, over time, are indicative of progression or regression in the heart disease. The time intervals and/or amplitudes are stored in a memory for later telemetry to an external receiver for review by medical personnel.
The determined time intervals may be time intervals between a heart sound and a morphological feature of a common cardiac cycle, such as an R-wave. The time intervals may alternatively or further be time intervals between different heart sounds of a common cardiac cycle.
The time intervals may more specifically be time spans between an R wave and an S1 heart sound and time spans between an S1 heart sound and an S2 heart sound. Also, the presence of an S3 heart sound may be indicative of elevated left ventricular diastolic pressure, as occurs in CHF patients. The presence of an S4 heart sound has been found to be indicative of hypertension, hypertrophic cardiomyopathy, cardiomyopathies, ischemia and/or myocardial infarction.
The time intervals and/or amplitudes are preferably determined for each cardiac cycle occurring over a time period, once each day, when the rhythm is being stable such as when the patient is at rest. The time intervals and/or amplitudes are then preferably averaged and the average time intervals stored in memory for later retrieval.
An increase in either time interval, over time, may be indicative of a progression in the heart disease. Conversely, a decrease in the time intervals may be indicative of a regression in the heart disease. To assure consistent data collection, the time intervals are preferably determined from heart activity while the patient is at rest and the cardiac signals are stable.
In accordance with a further aspect of the present invention, when the device is a cardiac stimulation device for delivering therapy, such as pacing therapy to the patient""s heart, the device itself may adjust stimulation therapy responsive to the determined physiological parameter measurements. The stimulation therapy adjustment may take the form, for example, of pacing rate adjustments to assist the patient in breathing or in the removal of fluid from the lungs.